The straight line distance Leeds-Bradford-Manchester is 37.6 miles, just 2 miles (5%) longer than Leeds-Manchester (35.6 miles). So a plan to link up the big cities of the north is hardly going to miss out Bradford which is 3/4 the size of Leeds, bigger than Hull and Newcastle and has the worst train services of any major city.
The "straight line distance" isn't too relevant unless you are planning to run in tunnels all the way - you'd need to factor in the dog-leg required to get from the Bradford Beck valley into the Aire valley (or whatever route it takes).
As for the idea of Bradford being bigger than Newcastle... we really need to get away from this definition of Bradford (which includes Ilkley/ Keighley etc) that appears much bigger than the city of Newcastle (which doesn't include anything on the south bank of the Tyne like Gateshead, doesn't include Wallsend or Longbenton... for example, there are only around a dozen "Metro" stations in Newcastle despite the continuous urban area sprawling on both banks of the river all the way along the Tyne Valley to the North Sea).
For me, they either build a proper fast link from Manchester to Leeds or they try to compromise by bringing in other bits of route - if you take the view that a "fast" line (doesn't need to be 200mph - an average speed of 100mph would be a huge step forward on what we have now!) is only delayed by a trivial amount of time for each mile diversion then there are a wide range of potential intermediate stops:
you could have Oldham (big town with no trains and poor links to the east)
you could have Brighouse (pretty much on a straight line from Manchester to Leeds, connections to local trains for Halifax/ Huddersfield/ Dewsbury etc)
you could have Wakefield (connections onto the ECML)
you could head further east to create a kind of "delta" junction with a Sheffield-Leeds line
you could have Huddersfield
...you could do a lot of things... I don't think Bradford is the only option - but at the moment we have a nice nebulous political idea that *could* be lots of things, so sounds great, until the moment you start to whittle down the options and disappoint people.